Safety-belt arrangement

ABSTRACT

A safety-belt arrangement for use in a motor vehicle has a safety-belt ( 5 ), one end of which is connected to a retractor ( 8 ), a sensor ( 11 ) generates a signal representative of the amount of safety-belt paid-out from the retractor. A force limiter ( 10 ) is associated with the retractor to permit safety-belt to be paid-out with a variable force limiting effect. A control arrangement ( 12 ) changes the level of force applied by the force limiter ( 10 ) by determining a reference point either by measuring a period of time following sensing of a predetermined parameter relating to a crash, and then determining when a specific length of safety-belt has been paid-out, or by sensing when the first level of resistive force is applied by the force limiter, and optionally then determining when a predetermined length of safety-belt has been paid-out, the control arrangement then measuring a predetermined period of time after the reference point has been reached before effecting the change of level of force applied by the force limiter ( 10 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a safety-belt arrangement, and moreparticularly relates to a safety-belt arrangement intended for use in amotor vehicle.

2. Description of Related Art

A simple safety-belt arrangement, as in widespread use, comprises alap-and-diagonal safety-belt arrangement, (or “three-point” safety-belt)with one end of the safety-belt being anchored to part of the vehicle,an intermediate part of the safety-belt being provided with a tonguereleasably engageable with a fixed buckle, and the other end of thesafety-belt, which may pass through a pillar-loop, being wound on to thespindle of a retractor. The retractor incorporates a mechanism whichlocks the spindle of the retractor to prevent further safety-belt beingpaid out from the retractor when an accident or a potential accidentsituation is sensed. If the safety-belt is locked in this way, however,the safety-belt can, in certain circumstances, apply a very greatretarding force to the occupant of the vehicle, decelerating theoccupant very swiftly, and also possibly injuring the occupant.

Thus it has been proposed to provide a force limiter in a safety-beltsystem, the force limiter being adapted to enable a certain length ofsafety-belt to be paid-out, with a force limiting or energy absorbingeffect, when very high forces are applied to the safety-belt.

It has been proposed to provide an arrangement in which the forcelimiting effect can be varied, at the start of or during an accidentsituation, in dependence upon the load required to be absorbed by thesafety-belt. An arrangement of this type is disclosed in WO 9749583-A inwhich the reel of a retractor is provided with a torsion elementarrangement which provides the force limiting effect. The torsionelement arrangement includes an axially extending torsion bar and aco-axial torsion sleeve. The torsion bar and the torsion sleeve eachterminate with a portion having a respective toothed periphery which canbe releasably engaged by a respective ratchet element. In this way thelevel of force of the force limiting effect can be selected, byselecting either the bar, or the sleeve, or both, to provide the forcelimiting effect.

It is desirable, in a typical accident situation, for the force limiterto provide a high force limiting effect during the initial stages of theaccident, as the occupant of the seat begins to move forwardly relativeto the chassis of the vehicle, and to provide a lower force limitingeffect at a subsequent stage in the accident when the occupant has movedforwardly and begins to impact with an inflating air-bag. It has,therefore, been proposed to have an arrangement in which the force level(or energy absorbing level) of the force limiter is adjusted after apredetermined period of time has elapsed following the sensing of anaccident or a potential accident situation, and thus after thepretensioner has been triggered. However, an arrangement of this typesuffers from various disadvantages.

Many accidents start with minor or short interaction with a firstobject, such as an impact with a car having a relatively low speed, oran impact with the edge of the road, or a crash barrier, with this minoror short interaction being followed, some moments later, by a severeimpact with a second object, such as an on-coming high speed car, a treeor a building. In such a situation the first interaction would be sensedby the sensor provided in the motor vehicle to detect an accident or apotential accident situation. The sensor may control various safetydevices within the motor vehicle, such as a pre-tensioner and anair-bag, but would also start the operation of the timer associated withthe force limiter.

In an accident of this type, the force limiter would exhibit a highlevel of force for a period of time following the minor or shortinteraction, but the occupant of the vehicle may not move forwardly atall, or may move forwardly only by a very short distance during thisperiod of time. Before the severe impact with the second object, thetimer may have timed-out, and thus the force limiter would only have thelower level of force. Then, when the severe impact occurs, which willcause the occupant to move forwardly, relative to the chassis, the levelof force provided by the force limiter might well be too low to absorball of the energy of the occupant before the occupant hits the steeringwheel or the dashboard.

SUMMARY OF THE INVENTION

The present invention seeks to provide an improved safety-beltarrangement.

According to this invention there is provided a safety-belt arrangementfor use in a motor vehicle, the safety-belt arrangement comprising asafety-belt, one end of which is connected to a retractor, a sensoradapted to generate a signal representative of the amount of thesafety-belt paid-out from the retractor, a force limiter associated withthe retractor adapted to permit the safety-belt to be paid-out with avariable force limiting effect, means to change the level of forceapplied by the force limiter, between a first level of resistive forceand a second level of resistive force, and a control unit to generate anoutput signal to control the means which change the level of force, thearrangement determining a reference point either by measuring a periodof time following sensing of a predetermined parameter relating to acrash, and then determining when a specific length of safety-belt hasbeen paid-out, or by sensing when the first level of resistive force issupplied by the force limiter and optionally then determining when apredetermined length of safety-belt has been paid-out and, after thereference point has been determined, measuring a predetermined period oftime after the reference point has been reached before said outputsignal is generated.

The predetermined parameter relating to a crash may be a deceleration ora predetermined deceleration, or some other equivalent parameterindicative of a crash, or a crash of a predetermined severity, isoccurring.

By changing the force level at the end of the predetermined period oftime, the energy absorbed will be adapted to the weight of the occupantand/or the violence of the crash.

Preferably the retractor incorporates a pretensioner.

In one embodiment of the invention responsive to the tension present inthe safety-belt are provided to determine the instant when the forcelimiter begins to provide the first level of resistive force.

In an alternative embodiment of the invention means responsive to apredetermined movement of components in the force limiter.

Preferably the force limiter is a torsion element force limiter and thedetermining means determine when a torsion element in the force limiteryields.

In a further alternative embodiment of the invention means responsive toa predetermined pay-out rate of safety-belt.

Conveniently the reference point is determined by sensing when the firstlevel of resistive force is applied by the force limiter and thendetermining when a predetermined length of safety-belt has beenpaid-out.

Advantageously a sensor is provided to determine the severity of thecrash.

Alternatively a sensor is provided to determine the severity of thecrash and the magnitude of the predetermined period of time is selectedin dependence on the sensed severity of the crash.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more readily understood, and so thatfurther features thereof may be appreciated, the invention will now bedescribed, by way of example, with reference to the accompanyingdrawings in which:

FIG. 1 is a diagrammatic figure illustrating one embodiment of theinvention,

FIG. 2 is a graphical figure showing a plot of force applied to asafety-belt against the length of belt withdrawn for a typical personprovided for purposes of explanation,

FIG. 3 is a graphical figure showing a plot of force applied to asafety-belt and length of belt extracted against time for a situationequivalent to that shown in FIG. 1, and

FIG. 4 is a further graphical figure corresponding to FIG. 3, butshowing a more detailed plot of length of belt extracted against time.

DETAILED DESCRIPTION OF THE INVENTION

Referring initially to FIG. 1 of the accompanying drawings, a vehicleseat 1 having a seat squab 2 and a back-rest 3 is illustrated. The seatis occupied by an occupant 4. The occupant 4 is illustrated wearing asafety-belt that forms a safety-belt arrangement in accordance with theinvention. The safety-belt comprises a webbing strap, one end of whichis anchored to part of the vehicle seat. Not shown in FIG. 1 is a tonguewhich is mounted on the safety-belt and which is engaged releasably in abuckle provided on the other side of the seat. Part of the safety-belt 5passes through a pillar-loop 7, and the other end of the safety-belt isconnected to a floor mounted retractor 8. The retractor 8 is adapted torespond to an accident situation to lock a spool within the retractor toprevent further safety-belt being paid-out from the retractor, bylocking in response to a sensed parameter such as a predetermineddeceleration of the vehicle and/or in response to the speed ofwithdrawal of the safety-belt from the retractor. The retractor isprovided with a pretensioner 9 which is adapted, in response to anaccident situation being sensed, to rotate the spool of the retractor towind in part of the safety-belt. Safety-belt arrangements having thefeatures described above are well known and in common use. However, inthis invention alternative mechanisms which lock the safety-belt inresponse to a sensed parameter may be used.

In the described embodiment the retractor 8 incorporates a variableforce limiter 10. The force limiter 10 may be a variable force limiterwhich provides a plurality of discrete force levels, such as a torsionelement force limiter of the type disclosed in WO 9749583-A, asdescribed above.

The retractor 8 may be provided with a sensor 11 adapted to sense theamount of safety-belt that has been withdrawn from the retractor.Various types of sensors which perform this function have been proposedbefore, and any of these sensors may be used. EP 0 723 895 A disclosesan arrangement in which a gear train is provided, driven by the spool ofa retractor, which causes the rotation of an element which has a partspiral outer edge. A sensor has a spring-biased probe which engages thepart spiral outer edge, so that as safety-belt is withdrawn from theretractor, the element with the part spiral outer edge rotates and thedegree of movement of the probe is indicative of the length ofsafety-belt that has been withdrawn. In another prior proposedarrangement, an element which co-rotates with the spool is provided witha plurality of evenly spaced markings which are sensed by an opticalsensor. The optical sensor generates a discrete pulse for each markingthat passes the sensor. Appropriate circuitry connected to the sensorcan determine the length of safety-belt that has been paid-out bydetermining the number of rotations, or part rotations, of the spool. AHall effect sensor can also be used to determine the rotations of thespool when the spool is provided with a number of appropriately locatedmagnets.

The output from the sensor 11 is provided to a central control unit 12.The central control unit 12, in the described embodiment, also receivessignals from other sources. The control unit 12 thus receives signalsfrom an accident sensor 13 in the form of an accelerometer. Theaccelerometer provides an output signal whenever deceleration in excessof a predetermined threshold is sensed. The accelerometer may alsoprovide a signal which indicates the severity of the crash. Such anaccident sensor 13 is conventional. The central unit 12 also receives asignal from a sensor 14 adapted to sense when the first level F₂ isreached. The sensor may sense the relative movement of components in theforce limiter 10, such as means which sense when a torsion elementyields. Alternatively, the sensor may sense the tension in thesafety-belt, or the speed of withdrawal of the belt from the retractor.

In the presently described embodiment, the control unit 12 is adapted tocontrol not only the pre-tensioner and the force limiter, but also anair-bag 15, which is located in front of the occupant.

Whilst, in the described embodiment, the single control unit 12 controlsa number of different items, in alternative embodiments of theinvention, the control function may be divided up between a number ofdifferent independent control units.

As will become clear from the following description, in a safety-beltarrangement in accordance with the invention, a variable force limitingeffect is provided, with the variation in the force limiting effectbeing controlled a predetermined time after a reference point has beenreached. The reference point may be a predetermined time after theinitial sensing of a crash, or a predetermined time after a specificdeceleration is reached during a crash, or when a specific predeterminedlength of safety-belt has been paid-out after either of thesepredetermined times. Alternatively the reference point may be reachedwhen a specific length of safety-belt has been paid-out, after a firstretarding force provided by the force limiter has been reached.Alternatively again, the reference point may simply be the point atwhich the first retarding force provided by the force limiter isreached.

Referring now to FIGS. 2 and 3, which is a graphical figure, theoperation of the safety-belt arrangement of FIG. 1 in a typical accidentfor a person of typical or normal weight will be described. The graph ofFIG. 2 is a plot of the force of the force limiting effect as plottedagainst the length of safety-belt withdrawn from the retractor. Thegraph of FIG. 3 is a plot of force and length of safety-belt withdrawnagainst time.

It can be seen that there are two predetermined retarding forces shownon the graph F₁ and F₂, the force F₂ being greater than the force F₁.These forces may, by way of example, be considered to be the forcesprovided by the torsion bar or the torsion tube, respectively, of anarrangement as shown in WO 9749583-A.

The accident situation commences at point 20 on the graph. At this stagea predetermined length of safety-belt has been withdrawn from theretractor. This is the length of safety-belt withdrawn as the occupantplaces the safety-belt in the position illustrated in FIG. 1. In thisexample this length of safety-belt is the length of safety-beltordinarily deployed when the occupant who is of normal or typical size,is sitting comfortably in an upright position, and is identified asL_(O). When an accident, or potential accident situation is sensed bythe accelerometer 13, for example, when the change in velocity of thevehicle exceeds a predetermined threshold, at time t₀, a signal from thecontrol unit 12 is passed to the pretensioner 9 which is activated towind in safety-belt on to the spool of the retractor 8. The spool of theretractor thus rotates and the safety-belt is wound in, against a gentlyincreasing resistive force which gradually rises to a level F_(p) untilthe point 21 is reached, when the safety-belt is fully tight around theoccupant. The length of belt withdrawn from the retractor is now at aminimum L_(min). The time is now t_(min). The period of time between t₀and t_(min) is a as shown in FIG. 3. This time period represents thetime taken for the pretensioner to pretension the webbing. For thepurposes of explanation, it will be assumed that the accident sensed bythe accelerometer 13 is an initial impact with the side of the road or acrash barrier. The deceleration of the chassis of the vehicle is onlyvery small, and thus the occupant does not move forward, relative to thechassis, by any noticeable distance as a consequence of the impact withthe side of the road or the crash barrier. However, since the accidenthas been sensed by the accelerometer, the safety-belt has been tensionedby the pre-tensioner, and the occupant has been pulled backwardly,relative to the chassis, by the pre-tensioner.

When the pre-tensioner has finished pulling the webbing strap, thetension force on the webbing will reduce and, due to the elasticity ofthe webbing strap, and also due to the elasticity of the clothes andbody of the occupant, webbing will be paid-out. Thus, clothes of theoccupant which were initially compressed during the pre-tensioning phasemay now re-expand. Thus the length of safety-belt paid-out increaseswhile the tension force on the safety-belt reduces.

As the webbing is paid-out, the locking mechanism of the retractor willoperate, and the retractor will become locked at time t_(lock) as shownat point 22 in FIG. 2. The period of time running from t_(min) tot_(lock) has a duration b as shown in FIG. 3.

At time instant t_(lock), the retractor becomes locked by the lockingmechanism that forms part of the retractor. Typically t_(lock) may be 15to 19 ms after t_(o). The precise time period between t_(o) and t_(lock)may depend upon factors such as variations in the weight of the seatoccupant, or the severity of the accident, but mainly upon the specificdesign and functions of the pre-tensioner.

t_(lock) may occur shortly before, simultaneously with, or shortly afterthe main impact of the accident presently being considered. During thismain impact the chassis of the vehicle is subjected to severedeceleration, and possibly also acceleration in the rearward direction,especially if the impact is an impact with a high speed vehicletravelling in the opposite direction.

During this phase of the accident, the webbing of the safety-belt actsto decelerate the occupant, and possibly acts to accelerate the occupantrearwardly, depending upon the movement of the chassis. During thisphase the force in the webbing is increased until the force reaches thehigher level F₂. When the force reaches the level F₂, the force limiterstarts to pay-out webbing.

The rate at which the webbing is paid-out is related to the integral ofthe relative acceleration (a_(rcl)) between the chassis and theoccupant. The relative acceleration (a_(rcl)) is the difference betweenthe acceleration of the chassis (a_(c)) and the acceleration of theoccupant (a_(o)). Thus:a _(rcl) =a _(c) −a _(o).The acceleration of the chassis is greater than or equal to theacceleration of the occupant. The acceleration of the occupant isrelated to the force level F₂ divided by the mass of the occupant. Ifthe mass of the occupant is large, a₀ may be small, but a_(rcl) may belarge. Thus, typically webbing is paid-out at a higher rate for aheavier occupant.

The time period between t_(lock) and the point where the resistive forceF₂ is reached, point 23 as shown on the graph, has a duration c, andthis time period is normally between 10 and 25 ms. This variation intime is typically due to the weight of the occupant, and the precisenature of the crash pulse, that is to say the period of time between thefirst impact and the main impact in the accident situation beingconsidered, and also the degree of deceleration or rearward accelerationapplied to the chassis to the vehicle in the particular crash situation.

It is thus to be understood that the period of time between t_(lock) andthe point 23, when the force F₂ is provided by the force limiter may,depending upon the precise circumstances, have a very differentduration, with a short period of 10 ms being experienced in somesituations, and a period of 100 ms being experienced in othersituations.

During this phase, t_(lock) and point 23, however, the occupant of theseat will only move forwardly very little, and mainly due to theelasticity of the webbing of the safety-belt.

The time when to change the force level of the force provided by theforce limiter from the initial high value F₂ to a lower value F₁ can bedetermined in different ways. To make the force limiter adaptive toweight of occupant, the length of safety-belt paid-out before change offorce level should be longer for a heavy person, and shorter for a lightperson.

This could be achieved by changing the force level a predetermined timeT₁ after a reference t_(ref2) related to the point 23, where the firstforce level is reached.

The point 23 could be indicated by the sensor 14 which senses when forceF₂ is reached.

The time period T₁ could start at point 23, but if a predeterminedlength L₁ is added before the start of T₁, the energy absorbed could bebetter adjusted to a desired dependence between energy absorbed andweight of occupant.

If, however, a certain length L₁ after point 23 is desired, then themeasuring of this length could start any time between t_(lock) at 22 andpoint 23. Such a reference, t_(ref1), could be found in a way that willnow be described.

During this phase, that is to say during the phase of movement withelastic stretching of the webbing following locking of the spool atpoint 22, and before the occupant of the seat has stretched the webbingto such an extent that the full resistive for F₂ of the force-limiter isexperienced at point 23, the sensor which senses the length ofsafety-belt that has been paid-out, takes a reference measurement attime t_(ref1) and effectively measures or sets a reference length ofsafety-belt that has been paid-out L_(ref1). This reference length isused as a start point when measuring the length L₁ of belt to bepaid-out against a specific resistive force F₂ as will now be described.

The point at which the reference measurement is made is shown as point24 on the graph of FIG. 2. In making the measurement, t_(ref1) to be aselected period of time, preferably within the time period of 20–30 ms,after t₀ to ensure that the time t_(ref1) will actually be in the timeperiod c following t_(lock), or, alternatively, the measurement at timet_(ref1) may be made in response to locking of the retractor reel.

If the occupant then moves forwardly, after the point 23 on the graph isreached, the resistive force F₂ is experienced as the webbing strap ofthe safety-belt is paid-out This is shown by the horizontal line 25 ofthe graph.

After a predetermined length L₁ of safety-belt has been paid-out, asmeasured from the reference length, L_(ref1), as shown at point L_(ref2)on the graph, identified as L_(ch), is reached when the force limiter 10no longer provides the high resistive force F₂, but instead provides thelower resistive force F₁, as shown at point 27 on the graph. This can bevisualised as changing from the torsion bar to the torsion tube of thearrangement of WO 9749583-A. Continued forward movement of the occupantis then effected against the much lower resistive force as shown at line28. In a typical case, during this phase of movement of the occupant ofthe seat, energy is also being absorbed, as shown in phantom at line 29,by an inflating air-bag.

It is to be appreciated that the change of the force level of the forcelimiter occurs when a certain amount of safety-belt has been paid-out,from the measured length L_(ref) and thus occurs when a certain amountof energy has been absorbed.

The energy absorbed is related to the length of belt withdrawnmultiplied by the force.

The safety-belt is withdrawn until a maximum extension L_(max) isachieved, at which point all of the kinetic energy of the occupant isabsorbed. L_(max) is selected so that the kinetic energy of the occupantis absorbed before the occupant actually reaches the steering wheel orwindscreen.

The transition between the high energy absorbing level F₂ and the lowenergy absorbing level F₁, as shown at 26 and 27 on the graph, iseffected when a predetermined length L_(ch) of safety-belt has been paidout. That predetermined length of safety-belt is determined by L₁ andT₁.

It is to be appreciated that during the final part of the withdrawal ofthe safety-belt, the occupant is restrained, not only by the safety-beltbut also by the air-bag, and thus the speed of withdrawal of thesafety-belt remains substantially constant, and the total retardingforce applied to the occupant remains substantially constant. FIG. 3shows, in phantom, at 30, the situation that would prevail if theair-bag did not inflate.

L₁ and T₁ may be adjusted in response to sensed crash conditions, suchthat L₁ and T₁ are related to the severity of the crash, and thus thedescribed arrangement can provide an optimum effect. Thus, the controlunit 12 will determine when a predetermined length (L₁) of safety belthas been paid out following t_(ref1) which is very shortly after lockingof the retractor reel and will then start an internal timer which timesa predetermined time period (T₁) so that the control signal that causesthe change of the resistive force from the higher level F₂ to the lowerlevel F₁.

FIG. 4 shows for a given severity of crash the length of safety-beltwithdrawn from the retractor reel plotted against time, andcorresponding to the lower part of FIG. 3, but showing alternatives fordifferent weights of occupant. The length of safety-belt withdrawn fromthe retractor falls following the commencement of the accident at t₀ atpoint 20 on the graph. At point 21 the safety-belt is fully tight aroundthe occupant of the vehicle, and the time period having duration aconcludes and the time period having duration b begins. During this timeperiod the occupant may move forwardly slightly in the seat and the timeperiod ends at point 22 on the graph as the retractor reel locks att_(lock). During the period of time following t_(lock) the occupantcontinues to be pushed forwardly applying tension to the webbing againsta restraining force that rises to the level F₂ during the time periodthat has a duration c. This time period is relatively short for a heavyperson, as exemplified by line 30, the time period being about 10 ms, asthe high restraining force is reached quickly following the locking ofthe retractor. The time period c for the heavy person identified by line30 terminates as shown by line 40, and then the safety-belt is pulledout against the retarding force F₂ and is identified as C_(min). Theline 50 shows the desired length L_(ch) for a heavy person.

Looking now at the example of a light person, as indicated by the line34, it can be seen that the time period that has the duration c isrelatively long, and does not end until the point indicated by the line44. This time period is identified as C_(max) The line 54 shows thedesired length L_(ch) for a light person.

The instant when the force limiter provides the force F₂ can occur atany time during the period t_(p), which is the period between the end ofC_(min) and the end of C_(max), and depends on various parametersincluding the weight of the occupant of the seat.

It can be seen that if the time is measured between the point at which apredetermined length of safety-belt L₁ after L_(ref) has been withdrawn,until the line representing the desired length L_(ch) is reached, ineach case the time is the same, namely T₁. It can also be seen that thesame effect is observed for occupants of decreasing weight as indicatedsuccessively by the lines 31, 32 and 33. The desired length L_(ch) foreach example are shown to be interconnected by the line 60.

Thus, at least to a good approximation, the length L_(ch) is reachedafter a first predetermined length L₁ of safety belt has been withdrawn,and a further length of belt has been withdrawn during a time period T₁.Consequently by determining when the length L₁ of safety-belt has beenwithdrawn, by determining t_(ref1) and then measuring the length ofsafety-belt as it is withdrawn from the retractor, and then measuringthe time T₁, to generate the control signal to effect the change of theresistive force it is not necessary to effect any measurement of anyparameter of the occupant related to the size of the occupant, thusminimising the number of sensors that need to be provided.

The pay-out rate will also be higher for a more sever crash, which makesthe system adaptive to the severity of the crash. If selected adaptivityfor weight and crash severity is desired the precise values of L₁ and T₁may be varied in dependence on the sensed severity of the crash.

Whilst embodiments have been disclosed in which the force limit levelcan be changed between two discrete levels, it is to be understood thatthe force limit level can be changed between a plurality of discretelevels. It is to be appreciated that the force limit level actuallyexhibited by the force limiter may be determined as a function of thecrash severity. Thus, in a very severe accident, an initial force limitlevel may be adopted which is a very high level, which can be visualisedas using simultaneously both the torsion bar and the torsion tube of WO9749583-A.

Thus, in the example given, an occupant of a seat may be assessed asbeing “light”, “normal weight” or “heavy”, and the described arrangementwill then function in the appropriate manner.

In the present Specification “comprises” means “includes or consists of”and “comprising” means “including or consists of”.

1. A safety-belt arrangement for use in a motor-vehicle, the safety-beltarrangement comprising a safety-belt, one end of which is connected to aretractor, a sensor adapted to generate a signal representative of theamount of the safety-belt paid-out from the retractor, a force limiterassociated with the retractor adapted to permit the safety-belt to bepaid-out with a variable force limiting effect, means to change thelevel of force applied by the force limiter, between a first level ofresistive force and a second level of resistive force, and a controlunit to generate an output signal to control the means which change thelevel of force, the arrangement determining a reference point either bymeasuring a period of time following sensing of a predeterminedparameter relating to a crash, and then determining when a specificlength of safety-belt has been paid out, or by sensing when the firstlevel of resistive force is supplied by the force limiter and optionallythen determining when a predetermined length of safety-belt has beenpaid-out and, after the reference point has been determined, measuring apredetermined period of time after the reference point has been reachedbefore said output signal is generated.
 2. An arrangement according toclaim 1 wherein the retractor incorporates a pretensioner.
 3. Anarrangement according to claim 1 wherein means responsive to the tensionpresent in the safety-belt are provided to determine the instant whenthe force limiter begins to provide the first level of resistive force.4. An arrangement according to claim 1 wherein means responsive to apredetermined movement of components in the force limiter are providedto determine the instant when the force limiter begins to provide thefirst level of resistive force.
 5. An arrangement according to claim 4wherein the force limiter is a torsion element force limiter and thedetermining means determine when a torsion element in the force limiteryields.
 6. An arrangement according to claim 1 wherein the referencepoint is determined by sensing when the first level of resistive forceis applied by the force limiter, and then determining when apredetermined length of safety-belt has been paid-out.
 7. An arrangementaccording to claim 6 wherein an additional sensor is provided todetermine the severity of the crash and the magnitude of thepredetermined length of safety-belt is selected in dependence on thesensed severity of the crash.
 8. An arrangement according to claim 1wherein an additional sensor is provided to determine the severity ofthe crash and the magnitude of the predetermined period of time isselected in dependence on the sensed severity of the crash.
 9. Asafety-belt arrangement for use in a motor vehicle, the safety beltarrangement comprising: a safety-belt having an end; a retractor capableof paying out the safety belt, the end of the safety-belt connected tothe retractor; a first sensor capable of generating a signalrepresentative of an amount of the safety-belt paid-out from theretractor; a force limiter associated with the retractor, the forcelimiter permitting the safety-belt to be paid-out with a variable forcelimiting effect, the force limiter having a torsion element for changingthe level of force applied by the force limiter between a first level ofresistive force and a second level of resistive force; and a controlunit capable of generating an output signal to control the torsionelement; wherein the arrangement is capable of determining a referencepoint in a series of collision events, and after the reference point hasbeen determined, the arrangement is capable of measuring a predeterminedperiod of time after the reference point has been reached before theoutput signal is generated.
 10. An arrangement according to claim 9,wherein the reference point is determined by measuring a period of timefollowing sensing a predetermined parameter relating to a crash and thendetermining when a specific length of safety-belt has been paid out. 11.An arrangement according to claim 9, wherein the reference point isdetermined by sensing when the first level of resistive force issupplied by the force limiter.
 12. An arrangement according to claim 11,wherein the arrangement is capable of determining when a predeterminedlength of safety-belt has been paid-out after sensing when the firstlevel of resistive force is supplied by the force limiter.
 13. Anarrangement according to claim 11, further comprising a second sensorcapable of responding to tension present in the safety-belt, the secondsensor thereby able to determine the instant when the force limiterbegins to provide the first level of resistive force.
 14. An arrangementaccording to claim 11, further comprising a second sensor capable ofresponding to a predetermined movement of components in the forcelimiter, the second sensor thereby able to determine when the forcelimiter begins to provide the first level of resistive force.
 15. Anarrangement according to claim 14, wherein the second sensor is able todetect when the torsion element in the force limiter yields.
 16. Anarrangement according to claim 11, further comprising a second sensorcapable of responding to a predetermined pay-out rate of thesafety-belt, the second sensor thereby able to determine when the forcelimiter begins to provide the first level of resistive force.